+1 on Rihanna Giddens, classically trained but increasingly more interested in the Roots music from her and her family's past.
I should mention Wailin' Jennys, three very good vocalists (and instrumentalists), their "live at the Mauch Chunk Opera" is an amazing album.
I am very partial to Katie Melua, not a huge voice but her version of Blues in the Nigh is stunning (and sly), with a great Hammond B3 organ in the background.
Shelby Lynne has a number of albums, beyond Just a little loving' that deserve some attention.
Melody Gardot is also interesting, I have both her "Worrisome Heart" and The Absence CDs, and like them both.
Of all of these, only Riannon Giddens has a huge voice, the others do a great deal with more limited instruments but fine sense of musicality.
None have a voice the size of Janis Joplin's, Aretha Franklin's or Linda Ronstadt (oh, how lucky we were)
Phil